1. Field of the Invention
This invention relates to a heat pipe, and in particular to a heat pipe including a monogroove slot designed to facilitate priming of the heat pipe in zero gravity.
2. Description of Related Art
The monogroove heat pipe is a heat transport device which includes parallel liquid and vapor channels connected to each other by a slot which communicates with an extends the length of both the liquid and vapor channels. During normal operation, liquid under pressure fills the liquid channel, the slot, and fine circumferential grooves which line the vapor channel walls. At the hot end of the heat pipe, liquid in the circumferential grooves evaporates, causing a flow of vapor in the vapor channel towards the cold end of the pipe, where condensation occurs. Liquid returns to the evaporator through the liquid channel by capillary forces, completing the fluid circuit. This type of heat transport device has a number of advantages, including simplicity, relative efficiency, and rapid start-up or priming, making the monogroove heat pipe suitable for a wide variety of applications. One application to which the monogroove heat pipe appears to be especially suited is the proposed U.S. Space Station.
Nevertheless, the monogroove heat pipe is disadvantageous in that, as the heat pipe is primed under zero gravity conditions, capillary forces tend to cause the liquid to preferentially fill the smaller slot region before filling the larger liquid channel, trapping enough vapor in the liquid channel to impede heat pipe start-up. This phenomenon is illustrated in FIG. 1, which shows a monogroove heat pipe 1, including a vapor channel 2, a slot 3, and a liquid channel 4. Due to launch loads, the liquid in the heat pipe may be forced into the condenser end 5 of the pipe during launch. The pipe is primed after launch by liquid entering liquid channel 4 in the direction of arrow A.
In addition to the working fluid liquid and vapor, the heat pipe may contain a small amount of non-condensable gas (NCG) (on the order of 30 ppm) due to a low grade reaction of the working fluid with the walls of the heat pipe.
During priming, slot 3 is designed to permit the vapor-NCG mixture to escape into vapor channel 2 under normal conditions but, under the zero gravity conditions noted above, capillary forces tend to cause a portion 6 of the slot 3 to prematurely fill with liquid, preventing escape of some of the vapor-NCG mixture into the vapor channel. As a result, the bubble 7 remaining in liquid channel 4 forms a barrier which prevents the liquid from completely filling the channel. Inadequate priming of the heat pipe prevents proper feeding of liquid into the unprimed portion of the evaporator, reducing heat transfer in that region.
In order to overcome this problem, it has been proposed to increase the slot dimension to be equal to one-half of the liquid channel diameter, thus eliminating the capillary action which causes the slot 3 to fill first. However, this solution severely reduces the axial heat transport capability of the heat pipe, and thus is not an acceptable solution.